As disclosed, for example, in non-patent document 1 as described below, a molded member including a tubular body and a flange formed at an end portion of the body is manufactured by performing a drawing process. The drawing process forms the body by stretching a base metal sheet, so that a thickness of a circumferential wall of the body is generally lower than that of the base material sheet.
The molded member molded by the drawing process as described above may be used as a motor case disclosed, for example, in patent document 1 and the like as described below. In this case, the circumferential wall of the body is expected to function as a shielding material for preventing magnetic leakage to the outside of the motor case. Depending on motor structures, the circumferential wall is also expected to function as a back yoke of a stator.
The performance of the body as the shield material or back yoke is improved as the thickness of the body increases. Therefore, when a molded member is produced by the drawing process as described above, a base metal sheet with a thickness larger than the required thickness of the circumferential wall of the body is selected taking into account the reduction in thickness of the body. However, the thickness of the base metal sheet is not always constant, and varies within an allowable range of the thickness called tolerance of thickness. Further, due to change of a state of a mold or variations in material properties, an amount of thickness reduction in the drawing process may also vary.
On the other hand, in order to reduce the vibration and noise of the motor, highly accurate circularity of an inner diameter may be required for the inner diameter of the motor case. Therefore, typically, in a step after a multi-stage drawing process, a finishing ironing process is performed on the body to improve the circularity of the inner diameter. The finishing ironing process is carried out using two molds in which an interval (clearance) of a gap between the two molds is set to be less than the thickness of the material of the body, when the ironing is applied by sandwiching the material of the body from both the inner side and the outer side using the two molds. The setting of the clearance to be less than the thickness of the material of the body refers to minus clearance.
At this time, when the thickness of the base metal sheet is thinner than a predetermined thickness or the thickness reduction rate is increased due to variations in the material properties of the base metal sheet or a change of a mold state during the drawing step, the thickness of the body prior to the ironing process will be less than the predetermined thickness. As a result, an amount of the ironing process becomes insufficient for the ironing mold prepared beforehand, so that the accuracy of the circularity of the inner diameter may be decreased. Conversely, when the thickness of the base metal sheet is thicker than the predetermined thickness, or the material properties of the base metal sheet varies and the state of the die state changes during the drawing step, or the like, the thickness of the body prior to the finishing ironing process may be too thicker than the predetermined thickness. In such a case, although the circularity of the inner diameter after the finishing ironing process is satisfied, another problem is caused that the base metal sheet adheres or seizes to the finishing ironing die.
The thickness of the circumferential wall of the body before the finishing ironing process varies due to variations in the thickness of the base metal sheet or variations in the thickness reduction rate during the drawing process. However, the clearance of the mold for carrying out the finishing ironing process is fixed, so that even if the thickness of the circumferential wall of the body before the finishing ironing process varies, the variation cannot be absorbed by changing conditions of the drawing process, thereby causing the above problems.
The problems are thus caused even if the thickness of the circumferential wall of the body before the finishing ironing process is thin or thick. Therefore, there have been severe requirements for the tolerance of thickness of the base metal sheet to be subjected to the multi-stage drawing process.
Thus, Patent Document 2 and the like as described below disclose a mold for performing compression drawing in the multistage drawing process in order to prevent a decrease in the thickness of the body of the drawn member.
In the mold for the compression drawing, a tubular member molded in a previous step is fitted into a deformation preventing member provided on a lower mold in a state where an opening flange of the tubular member is placed downward, the opening flange is positioned onto a concave portion of a plate provided at the lower mold, and an outer periphery of the flange is engaged with the concave portion. An upper mold is then allowed to descend to press-fit a tubular portion of the tubular member into a hole of a die provided at the upper mold, whereby compressive force acts and the compression drawing process is carried out.
In this case, since the deformation preventing member can move upward and downward relative to the plate, a side wall of the tubular member is mostly not subjected to any tensile force, so that a decrease in the thickness is suppressed, and it is rather possible to increase the thickness (an increase in thickness).
It should be understood that the compressive force thus applied to the body element is equal to deformation resistance of the body element when press-fitted into the hole of the die. In other words, contributed to the increase in the thickness are mold clearance between the die and the punch, which is mainly relevant to the deformation resistance, a curvature radius of a shoulder portion of the die, and material strength (yield strength/cross sectional area) of the body element.